wifi: wcn36xx: Annotate struct wcn36xx_hal_ind_msg with __counted_by

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time checking via CONFIG_UBSAN_BOUNDS
(for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

As found with Coccinelle[1], add __counted_by for struct wcn36xx_hal_ind_msg.

[1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci

Cc: Loic Poulain <loic.poulain@linaro.org>
Cc: Kalle Valo <kvalo@kernel.org>
Cc: wcn36xx@lists.infradead.org
Cc: linux-wireless@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20230915200622.never.820-kees@kernel.org

drivers/net/wireless/ath/wcn36xx/smd.h

@@ -47,7 +47,7 @@ struct wcn36xx_fw_msg_status_rsp {
 struct wcn36xx_hal_ind_msg {
 	struct list_head list;
 	size_t msg_len;
-	u8 msg[];
+	u8 msg[] __counted_by(msg_len);
 };
 
 struct wcn36xx;